Throw grenades at all of the red barrels in the windows around the map.

1: by the in the window by the Double Barrel

2: In the window Next to Wonder Fizz

3: In the window outside of the MP-40 Teleporter

4: In the window under Quick Revive

5: In the window that is used for the Fly Trap

6: In front of the big door by the hanging guy in the window at the type 100 Teleporter

7: In the window in the room next to the Type 100

There was a glitch we were getting in our game, where when the grenade would upgrade, it would take one of our guns at random. We found a fix.

PS: After you throw the grenade at the last barrel to upgrade, once it blows up, wait 1 second and hold a grenade. Throw it away once the Barrel Explodes, so you don't go down. This fixed it for us, but I don't know if it is happening for everyone.